1. Field of the Invention
This invention relates to apparatus and method for high vacuum pump systems, and particularly to those wherein a chamber evacuated by the pump must be repeatedly vented to an atmosphere that contains substantial water vapor. Deterioration of the efficiency of the vacuum pump operation, due to contamination of the pump oil by accumulation of condensed water vapor, is prevented by the invention herein disclosed.
2. Description of the Prior Art
In commercial metallization or coating operations, the problem of water accumulation in the oil of mechanical vacuum pumps, and particularly in second or final stage high vacuum diffusion vacuum pumps, and consequent loss of pumping efficiency, has been a recognized problem for years. The high cost of special pump oils employed for lubrication and operation of high vacuum pumps makes it economically prohibitive to replace that oil frequently in order to maintain maximum pumping efficiency. This is more especially true in the case of silicone oil used in the diffusion pumps, which is extremely costly. The recycle rate of processing workpieces in a vacuum metallizing chamber suffers with deterioration of the vacuum pumping efficiency, often being reduced under high humidity atmospheric conditions to one-half to one-tenth that of which the system is capable when operating at maximum efficiency.
One of the solutions to the problem proposed by the prior art has been the incorporation of cryopumps in conjunction with a diffusion pump and/or mechanical forepumps in order to extract vapor present in the vacuum duct as frost on the cryo surface. For example, very low temperature liquid nitrogen or helium coldtraps which may be of optical dense design such as chevron baffle form to increase their trapping ability, or cryocoils such as Meissner coils, have been used for this purpose. The operating costs of these systems are relatively high and the commercial success has been variable. The problem still remains of what to do with the frost on the cold trap when it builds up to a point where the trap is no longer effective. These problems are especially acute with systems using liquid nitrogen as refrigerant which introduces special disadvantages in terms of refrigerant handling problems, maintenance work, personnel safety risks, as well as the high costs. Alternate cascade refrigerant systems of the Freon/ethylene type have also been employed, and while these eliminate the high risk to operating personnel of the liquified nitrogen systems, still they have not solved the water contamination problem spoken of above because the accumulated frost must still be eliminated periodically and contamination of the pumps in the process remains.
U.S. Pat. Nos. 3,168,819, 3,485,054, 3,512,369, 3,536,418, 3,712,074 and 4,148,196 all disclose cryopumps in conjunction with a diffusion pump, and represent the most pertinent prior patent art of which the inventor is aware. Of these, U.S. Pat. No. 3,485,054 is probably most relevant to this invention but does not suggest the solution disclosed herein. The patent art alternately suggests other approaches to handling some of the problems mentioned above, for example special mechanical improvements in vacuum chamber sealing arrangements, as disclosed in U.S. Pat. No. 3,095,494; or product mounting arrangements in the vacuum chamber, as disclosed in U.S. Pat. No. 4,191,128. On the specific subject of improving the production rate under high humidity conditions of vacuum metallizing operations, the most pertinent disclosure known to the inventor is contained in a technical paper dated December 1977 distributed by Polycold Systems, Inc. of San Rafael, Calif. entitled "Improving Summer Pumpdowns in Vacuum Coating Systems". This describes several systems incorporating combinations of cryopumps assisting diffusion and mechanical pump systems, and provides a discussion of specific problems encountered in vacuum metallizing operations. The disclosure includes reference to "hot gas" defrost of a Meissner coil made practical by a cascade refrigeration system. The publication reports that practical and economic improvements are achieved in combining a cryopump with a diffusion pump but so far as is known, this publication has still not led to a satisfactory solution of the problems of water contamination of the pump oil and resultant decrease in operating efficiency.